For navigation of ships, a method of detecting the presence of target objects around a ship (hereinafter, referred to as a “ship concerned”) using a radar apparatus to monitor positional relations between the target objects and the ship concerned is effective for avoiding collisions with other ships. Particularly, the use of the radar apparatus becomes essential in cases of poor visibility such as at nighttime and when dense fog appears. The aforementioned type of radar apparatus may be equipped with an image processing device which displays images by appropriately processing information based on echo signals.
However, if an operator is unfamiliar with the radar images, then there have been cases where it is difficult to determine whether the sources of the echoes on the screen (displayed image showing the target objects) interfere with the navigation. On the other hand, even if the operator is familiar with the radar images, it is difficult to determine the risk of collision unless the operator looks at the screen for at least several scans.
Therefore, a radar apparatus equipped with Automatic Radar Plotting Aids (ARPAs), which assist in the use of the radar apparatus, may be used. The ARPAs have a function to calculate a course of another ship and a time its takes for the another ship to approach the nearest vicinity of the ship concerned based on information such as a relative position and a relative orientation of the ship concerned and the another ship obtained from the radar apparatus in real time. By displaying the calculation results on the screen, the information which is necessary to avoid collisions is provided for the operator so as to be visually easy to understand. JP2003-48595(A) discloses a radar apparatus equipped with this type of ARPAs.
JP2003-48595(A) discloses an other-ship display device configured to display with high visibility for ships having a high risk of collision and low visibility for ships having a low risk of collision. JP2003-48595(A) describes a display device capable of displaying a high visibility image which aids in avoiding collisions, which is the objective of this kind of device.
Further, there is another radar apparatus, which has a so-called trail function to display a current position and a trail (trace of movement) of another ship (target object) on the screen. Conventionally, a relative trail display mode which displays a relative change in position with respect to the ship concerned and an actual trail display mode which displays an absolute change in position, are known as the display modes of the aforementioned trail. In the relative trail display mode, the relative change in position of another ship with respect to the ship concerned is displayed as the trail on the screen. In the actual trail display mode, the actual change in position of the target object with respect to a stationary target object such as land appears on the screen as the trail, regardless of the movement of the ship concerned. The operator selectively uses these trail functions according to the situations and preferences, and uses it to predict the moving directions of other ships or to avoid collisions and the like.
The ARPAs disclosed in JP2003-48595(A) can display an intuitive and easy-to-understand on-screen information to avoid collisions with other ships. However, since technologically advanced processing, such as the acquisition and tracking operations of the target objects, are needed, internal processing becomes complex; thus, leading to large-scale circuits. Therefore, there is room for improvement in terms of lower manufacturing costs.
Further, even when the trail function is displayed, the following problems are present due to the display modes. That is, in the relative trail display mode, the relative change in position of the target object with respect to the ship concerned is configured to be displayed by the trail; therefore, when the ship concerned is moving, even the stationary target objects which are not moving in reality are displayed as the trails. For this reason, it is difficult to determine whether the target object of interest is a moving target object or a stationary target object. In this regard, in the actual trail display mode, since the absolute change in position of the target object is configured to be displayed by the trail, even when the ship concerned is moving, the stationary target object such as land are not displayed as the trail. However, in the actual trail display mode, since the changes in actual position of other ships are displayed, it is difficult for the operator to intuitively determine if other ships are approaching the ship concerned like the relative trail display mode.